Projection objective of a microlithographic projection exposure apparatus

ABSTRACT

A projection objective of a microlithographic projection exposure apparatus has a high index refractive optical element with an index of refraction greater than 1.6. This element has a volume and a material related optical property which varies over the volume. Variations of this optical property cause an aberration of the objective. In one embodiment at least 4 optical surfaces are provided that are arranged in at least one volume which is optically conjugate with the volume of the refractive optical element. Each optical surface comprises at least one correction means, for example a surface deformation or a birefringent layer with locally varying properties, which at least partially corrects the aberration caused by the variation of the optical property.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/330,980, filed Dec. 9, 2008, which is a continuation of internationalapplication PCT/EP2007/005297, filed Jun. 15, 2007, which claims benefitof U.S. provisional application Ser. No. 60/814,385 filed Jun. 16, 2006.The full disclosure of these applications are incorporated herein byreference.

FIELD

The disclosure generally relates to a projection objective of amicrolithographic projection exposure apparatus, as well as relatedcomponents, apparatus and methods. Such apparatus can be used, forexample, in the production of integrated circuits and othermicrostructured components. As an example, the disclosure relates to aprojection objective having a refractive optical element that has amaterial related optical property (e.g. the refractive index or thebirefringence) which varies over the volume of the element.

BACKGROUND

Integrated electrical circuits and other microstructured components areconventionally produced by applying a plurality of structured layersonto a suitable substrate which, for example, may be a silicon wafer. Inorder to structure the layers, they can be first covered with aphotoresist which is sensitive to light of a particular wavelength, forexample 248 nm, 193 nm or 157 nm. The wafer coated in this way can besubsequently exposed in a projection exposure apparatus. During theexposure, a pattern of structures on a mask can be projected onto thephotoresist with the aid of a projection objective. Since the imagingscale is generally less than 1, such projection objectives are oftenreferred to as reduction objectives.

After the photoresist has been developed, the wafer can be subjected toan etching process so that the layer becomes structured according to thepattern on the mask. The photoresist still remaining can then be removedfrom the other parts of the layer. This process can repeated until allthe layers have been applied on the wafer.

SUMMARY

In some instances, it is desirable to provide a projection exposureapparatus designed to lithographically define structures with smallerand smaller dimensions on the wafer. Small structures can lead to highintegration densities, which can generally have a favorable effect onthe performance of the microstructured components produced with the aidof such apparatus.

In many instances, the minimum size of the structures depends primarilyon the resolution of the projection objective. Because the resolution ofprojection objectives is generally proportional to the wavelength of theprojection light, one way of decreasing the resolution can be to useprojection light with shorter and shorter wavelengths. Often, theshortest wavelengths currently used are in the deep ultraviolet (DUV)spectral range and are equal to, for example, 193 nm, or occasionallyeven 157 nm.

In certain instances, the resolution is inversely proportional to thenumerical aperture on the object side of the projection objective. Insuch instances, a way of decreasing the resolution can be based on theidea of introducing an immersion liquid with a high refractive indexinto an immersion space, which remains between a last optical element onthe image side of the projection objective and the photoresist oranother photosensitive layer to be exposed. Projection objectives whichare designed for immersed operation are commonly referred to asimmersion objectives, and can often achieve numerical apertures of morethan 1, for example 1.3 or 1.4.

Amorphous quartz glass or calcium fluoride (CaF₂) is conventionally usedas a material for the last optical element on the image side ofimmersion objectives. At a wavelength λ=193 nm, quartz glass has arefractive index of approximately 1.56, and CaF₂ has a retractive indexof approximately 1.50. Because the refractive index of the last opticalelement on the image side can limit the numerical aperture of theimmersion objective, the use of materials with an even higher refractiveindex is being considered particularly for the last lens on the imageside of the projection objective. Certain fluorides such as bariumfluoride (BaF₂) or lanthanum fluoride (LaF₃), certain chlorides such assodium chloride (NaCl) or potassium chloride (KCl) or certain oxidessuch as magnesium spinel (MgAIaO₄), calcium spinel (CaAl₂O₄), yttriumaluminium garnet (Y₃Al₅Oi₂) or magnesium excess spinel (MgOOAl₂O₃) areenvisaged, for example. However, many issues may still need to beaddressed with respect to producing and processing such high-indexoptical materials. As an example, currently the homogeneity of certainoptical properties of these materials (e.g., refractive index,birefringence, absorption and/or scattering) may be inferior to those ofthe amorphous and crystalline lens materials that are commonly used.Birefringence in the last optical element on the image side can be ofparticular interest because the projection light passes through thiselement with a particularly wide angle spectrum. But, in high-resolutionimmersion objectives, inhomogeneous and anisotropic optical propertiesin the last lens on the image side can cause undesirable aberrations, sothat it may not be practical to readily use the new high-indexmaterials.

In some aspects, the disclosure provides a projection objective of amicrolithographic projection exposure apparatus in which aberrationscaused by high-index optical materials are reduced.

In certain embodiments, the disclosure provides a projection objectiveof a microlithographic projection exposure apparatus that has a highindex refractive optical element with an index of refraction greaterthan 1.6 at a wavelength of 193 nm. This optical element has a volumeand a material related optical property which varies over the volume.Variations of this optical property cause an aberration of theobjective. Optionally, at least 4 (e.g., at least 6, at least 8) opticalsurfaces are provided that are arranged in one continuous volume (or aredistributed over a plurality of distinct volumes) which is opticallyconjugate with the volume of the refractive optical element. Eachoptical surface includes at least one correction mechanism (e.g., asurface deformation or a birefringent layer with locally varyingproperties) which at least partially corrects the aberration caused bythe variation of the optical property.

The disclosure is based, at least in part, on the idea that a spatiallyinhomogeneous optical property can be corrected successfully byproviding a spatially well-resolved conjugate volume, in which asuitable correction mechanism are arranged.

In order to determine the position and placement of the correctionmechanism, the refractive optical element may initially be subdividedconceptually into a large number of small volume elements. In anotherstep, the relevant optical property (or several optical properties) isdetermined for the volume element. In a further step, the place wherethese volume elements are imaged into another portion of the objectiveis determined, and an overall volume conjugate with the volume of therefractive optical element is thus determined.

If the refractive optical element is the last element on the image side,there is generally at least one conjugate volume between theillumination system and the mask plane. In such instances, it may bepossible to achieve only a limited, angle-independent correction ofaberrations which are caused by inhomogeneous refractive indexdistributions. It may therefore be desirable to have at least oneintermediate image in the projection objective. In front of such anintermediate image, there is a further conjugate volume in which thecorrection mechanism can be arranged. Those aberrations, which arecaused by inhomogeneities of angle-dependent optical properties, canthen also be corrected by the correction mechanism.

A correction mechanism associated with a correction element is thendetermined so that it at least partially corrects a component of anaberration which is caused by the considered volume element in the lastoptical element.

In general, it may not be possible to accommodate an arbitrarily largenumber of optical surfaces in the volume conjugate with the refractiveoptical element. Therefore an optimization process may be carried out. Aresult of such a process may be that only a few surfaces having acorrection mechanism remain that at least substantially correct theaberrations due to the inhomogeneities in the refractive opticalelement.

A high-index refractive optical element is defined in this context as amaterial having a refractive index at the wavelength of λ=193 nm of morethan 1.6. For refractive elements having a refractive indexsignificantly above that value, for example greater than 1.8 or evengreater than 2.0, the present disclosure can be even more advantageousbecause such materials often have even greater variations of certainoptical properties that cause aberrations in the projection objective.

In many cases, such a high-index refractive optical element is the lastoptical element of the objective, because, when the optical element isso positioned within the objective, it can have a particularlyadvantageous effect on the numerical aperture NA of the projectionobjective. Typically, the refractive optical element then has at leastone curved surface, often on its object side. If the objective isdesigned for immersion operation, during which an immersion liquid atleast partially covers a photosensitive layer which is arranged in animage plane of the objective, the refractive optical element may contactthe immersion liquid during the immersion operation.

If the undesired variations of the optical property are distributed overthe entire volume of the refractive optical element, it may beadvantageous to have conjugate surfaces arranged in the volume of therefractive index that are spaced apart (e.g., by less than 5 mm, by lessthan 2.5 mm) in a direction parallel to an optical axis of theobjective. This can help ensure sufficient spatial resolution so that adefect in the material of the refractive optical element which hasdimensions in the millimeter range can be successfully addressed.

The inhomogeneous optical properties considered here include, but arenot limited to, the refractive index, the birefringence, the degree ofabsorption and/or the amount of scattering.

In order to correct a wavefront deformation which is caused by aninhomogeneous refractive index distribution in the refractive element,at least one of the optical surfaces may have a correction mechanismformed as a non-axis symmetric deformation of the at least one opticalsurface. The deformation can be configured to correct a wavefrontdeformation associated with the aberration. Such a surface deformationmay be produced by locally applying a material to the at least oneoptical surface, and/or by locally ablating application material fromthe at least one optically surface.

In order to correct a spatially inhomogeneous birefringence, it isgenerally desirable for the correction mechanism to modify the state ofpolarization of light passing through it. To this end the correctionmechanism may include structures made of a birefringent material, suchas layers or plates having thicknesses that vary locally over theoptical surface that having the at least one correction mechanism.Additionally or as an alternative, it is possible to useform-birefringent structures for the at least one correction mechanism.

If the inhomogeneous optical property is the degree of absorption, forexample, the at least one optical surface may have a correctionmechanism formed by a portion of the at least one optical surface, or avolume adjacent to the at least one optical surface, having a locallyvarying transmissivity or reflectivity.

If the inhomogeneous optical property is the amount of scattering light,for example, the at least one optical surface may have a correctionmechanism formed by a portion of the at least one optical surface, or avolume adjacent to the at least one optical surface, having a locallyvarying scattering effect. For example, within a portion of thehigh-index refractive optical element the scattering may be higher thanin surrounding portions. The correction mechanism may then be formed bya surface which has its smallest scattering effect in an area which isoptically conjugate with the portion within the refractive opticalelement where an increased scattering occurs. In total, this wouldachieve a compensating effect. Different degrees of scattering may beproduced, for example, by providing an optical surface having a locallyvarying surface roughness.

If the objective has N intermediate image surface and N+1 pupilsurfaces, with N=0, 1, 2, . . . , the at least optical surface (e.g.,the at least 4 optical surfaces) may be separated by k intermediateimage surfaces and k pupil surfaces, with k=0, 1, 2, . . . , N.

This can help ensure that light bundles passing through the conjugatevolume elements are not inverted by an odd number of pupil orintermediate image planes.

In some embodiments, several different types of correction mechanism canbe used. The different types of correction mechanism can correctaberrations that are generated by inhomogeneities of several or all ofthe aforementioned optical properties, and/or optical properties thatare not explicitly mentioned here.

In principle, the optical surfaces including the correction mechanismmay be formed on supports that have virtually any axisymmetric shape.Nevertheless, it can be particularly advantageous for one, several orall of the surfaces to be formed on plane-parallel plates. The platesmay have different thicknesses and different distances from one another.Some or all of the plates may be arranged so that they are displaceablealong an optical axis of the projection objective.

In some embodiments, it can be particularly desirable to form thesurfaces on plane-parallel plates because during the optical design ofthe immersion objective it is possible to provide just one single thickplate initially, which is divided into a plurality of individual platesduring the subsequent optimization. The division into a plurality ofoptionally displaceable individual plates does not change the opticaleffect, or changes it only slightly, so that the other optical elementsof the immersion objective do not need to be adapted, or need to beadapted only slightly. Furthermore, non-axisymmetric surfacedeformations, which are suitable for the correction of wavefrontdeformations, can locally be produced particularly advantageously onplane-parallel plates.

Optionally, a plurality of thinner plates may be provided. Such platescan be displaced along the optical axis without significantly affectingthe optical properties of the projection objective.

In both cases, the design of the projection objective is greatlyfacilitated, because it is possible to start with an initial designincluding one single thick plate, or a plurality of thinner plates, andto position the thinner plates (in the case of the thicker plate afterconceptually dividing it into two or more thinner plates) at theappropriate axial positions without altering the initial designotherwise.

In order to allow adaptation to different operating states, for exampledifferent illumination angle distributions or different masks, one ormore plates may be held in an exchange holder. Because both theillumination angle distribution and the mask usually affect thepositions where the light rays pass through the refractive opticalelement, when changing the illumination angle distribution and/or themask it may be expedient to employ plates whose correction mechanism arespecially adapted to the portions of the refractive optical elementthrough which the projection light actually passes. Furthermore, theoptical properties of the refractive optical element may change as aresult of photoinduced degradation phenomena in the course of operatingthe apparatus, so that adaptation of the corrective effect may likewisebe desirable.

The optical surfaces including the correction mechanism(s) do notnecessarily have to be arranged adjacent to one another. In many casesit will be more advantageous to separate the surfaces by at least onelens or other optical element which does not correct the aberration(s).

In certain embodiments, the disclosure provides a method of designing aprojection objective of a microlithographic exposure apparatus. Themethod includes:

-   -   a) determining an initial design of the objective, wherein the        initial design includes        -   a refractive optical element and        -   at least 2 transparent plane-parallel correction plates            which can be conceptually shifted along an optical axis of            the objective such that they are at least partially arranged            in a conjugate volume which is devoid of any optical            elements and which is optically conjugate with the total            volume of the high index refractive optical element;    -   b) determining a volume element in the refractive optical        element in which a material related optical property varies,        wherein a variation of the optical property in the volume        element causes an aberration;    -   c) determining a conjugate volume element which is optically        conjugate to the volume element determined in step b) and which        is located in the conjugate volume being devoid of any optical        elements;    -   d) conceptually positioning at least one of the correction        plates such that a surface of the at least one plate is arranged        in the conjugate volume element determined in step c); and    -   e) designing a correction mechanism at the surface of the at        least one correction plate that at least partially reduce the        aberration.

It should be noted that the least 2 transparent plane-parallelcorrection plates mentioned in step a) may also be conceptuallyconsidered as forming a single thicker plane-parallel correction platewhich may be divided in two ore more individual plates as desired.

The details of one or more embodiments of the disclosure are set forthin the accompanying drawings and the description below. Other featuresand advantages of the disclosure will be apparent from the descriptionand drawings, and from the claims.

DESCRIPTION OF DRAWINGS

Various features and advantages of the disclosure may be more readilyunderstood with reference to the following detailed description taken inconjunction with the accompanying drawing in which:

FIG. 1 is a schematic meridional section through a projection exposureapparatus having a projection objective;

FIG. 2 is a highly schematic meridional section through the projectionobjective shown in FIG. 1;

FIG. 3 is a side view of a correction device that may be arranged in theprojection objective shown in FIG. 2;

FIG. 4 is a side view of a correction device that may be arranged in theprojection objective shown in FIG. 2;

FIG. 5 is a side view of a correction device that may be arranged in theprojection objective shown in FIG. 2;

FIG. 6 is a schematic meridional section through a projection objectivehaving two intermediate image surfaces;

FIG. 7 is a schematic meridional section through a projection objectivesimilar to the objective shown in FIG. 6, but with a differentarrangement of correction plates;

FIG. 8 is a meridional section through a projection objective having twointermediate image surfaces and three mirrors;

FIG. 9 is a meridional section through a projection objective having twointermediate image surfaces and two mirrors;

FIG. 10 is a schematic meridional section through a projection objectiveconfigured to illustrate the concept of conjugate planes;

FIG. 11 is a schematic meridional section through a portion of aprojection objective configured to illustrate the imaging of conjugatevolumes within the projection objective;

FIG. 12 are graphs showing how the optical transmission contrast inconjugated volumes shown in FIG. 11 decreases.

DETAILED DESCRIPTION

FIG. 1 is a schematic meridional section through a microlithographicprojection exposure apparatus which is denoted in its entirety by 10.The projection exposure apparatus 10 includes an illumination system 12for generating projection light 13, which includes a light source 14,illumination optics indicated by 16 and a field stop 18. In theembodiment shown the projection light has a wavelength of 193 nm. As amatter of course, other wavelengths, for example 157 nm or 248 nm, arealso contemplated.

The projection exposure apparatus 10 furthermore includes a projectionobjective 20 which contains a plurality of optical elements such aslenses, mirrors or filter elements. For the sake of simplicity, theprojection objective 20 is shown with only three lenses L1, L2 and L3;more realistic embodiments of projection objectives are shown, forexample, in FIGS. 6 and 7. The projection objective 20 is used to imagea mask 24, which is arranged in a mask plane 22 of the projectionobjective 20, onto a photosensitive layer 26 which, for example, mayconsist of a photoresist. The layer 26 is arranged in an image plane 28of the projection objective 20 and is applied on a wafer 30.

In this embodiment the support 30 is fastened on the bottom of atrough-like, open-topped container 32 which can be displaced (in a waywhich is not represented in detail) parallel to the image plane 28 withthe aid of a displacement device. The container 32 is filled with animmersion liquid 34 so that the projection objective 20 is immersed withits last lens L3 on the image side into the immersion liquid 34 duringoperation of the projection exposure apparatus 10.

Via a feed line 36 and a discharge line 38, the container 32 isconnected to a treatment unit 40 which (e.g., in a manner which is knownand therefore not represented in detail) contains a circulating pump, afilter for cleaning the immersion liquid 34 and a temperature controlunit. It should be understood that other setups for immersing theprojection objective 20 may be used instead. For example, the immersionliquid 34 may not be contained in a container, but may be directlyreleased on and sucked off the photosensitive layer 26 (e.g., in amanner which is known).

FIG. 2 shows the projection objective 20 of FIG. 1 in an enlargedschematic representation. There it can be seen that the lens L1 imagesthe mask plane 22 onto an intermediate image plane 42. The image of themask 24 formed in this intermediate image plane 42 may be affected bysignificant aberrations. The lenses L2 and L3 image the intermediateimage 42 onto the image plane 28. The mask plane 22, the intermediateimage 42 and the image plane 28 are therefore optically conjugate withone another.

Light rays which emerge from a point of the mask plane 22 converge viathe aberration-affected intermediate image at a point in the image plane28.

The last lens L3 on the image side is a plano-convex lens in theembodiment shown; other lens shapes, for example convex-concave or evenplane-parallel, are of course also possible. In some embodiments, thelast lens L3 on the image side is made of magnesium spinel (MgAl₂O₄).

It is believed that this is a lens material which, owing to insufficientoptical homogeneity and purity, cannot yet be used in such projectionobjectives without the correction mechanism proposed hereinafter. One ormore optical properties of the lens L3 therefore vary—albeitslightly—over the volume of the lens L3. This optical property may, forexample, be the refractive index Inhomogeneous refractive indexdistributions, which are also referred to as schlieren when they have aparticularly short-wave profile, in turn cause wavefront deformationsfor the projection light passing through. In the case of opticallyanisotropic materials, a refractive index may still be defined as ascalar quantity, for example as an average value between the ordinaryrefractive index and the extraordinary refractive index.

In the case of optically anisotropic and therefore birefringent lensmaterials, the birefringence tensor may furthermore be a function of theposition, so that equally polarized and mutually parallel raysexperience a different change in their polarization state as a functionof the position where they pass through the lens L3.

It is also possible that the lens L3 is not homogeneously transparent.Lens L3 can have a spatially varying transmission coefficient, orlocally varying scattering properties.

The effect of all the inhomogeneities mentioned above is that theimaging of the mask 22 on the photosensitive layer 26 is perturbed byaberrations.

In order to correct these aberrations, a correction device 44, whichincludes four correction elements 46 a, 46 b, 46 c, 46 d in thisembodiment, is arranged between the lens L1 and the intermediate imageplane 42. The correction elements 46 a, 46 b, 46 c, 46 d areplane-parallel transparent plates, which can be rendered highlyantireflective for the working wavelength of the projection exposureapparatus 10. As an alternative to this, the correction elements 46 a,46 b, 46 c, 46 d may also adjoin a liquid on one or both sides, in orderto reduce undesired light reflections. Details of the structure of thecorrection elements 46 a, 46 b, 46 c, 46 d will be explained in moredetail below with reference to FIG. 3.

The correction elements 46 a, 46 b, 46 c, 46 d of the correction device44 are indicated inside a volume L3′, which is conjugate with the volumeof the last lens L3 on the image side.

The thickness, the placement and the design of the correction elements46 a, 46 b, 46 c, 46 d may be determined in accordance with thefollowing method:

First, the optical properties of the last lens L3 on the image side aremeasured with three-dimensional position resolution. This measurementmay for example be carried out on a cylindrical lens preform, onto whichthe lens geometry is transferred computationally. During the subsequentproduction of the lens and the computational transfer of the dataobtained with the aid of the preform onto the lens, particular attentionis to be paid to the azimuthal placement and orientation of the outerfaces of the crystal. A spatial accuracy of less than 100 μm between themeasurement data from the crystal, on the one hand, and the productionof the lens and the replication in the computer, on the other hand, isexpedient. Suitable methods for this are of the tomographic type. Are-view of these can be found in the book by A. C. Kak and M. Slaneyentitled “Principles of Computerized Tomographic Imaging”, IEEE press,New York, 1987, which is also published on the Internet athttp://www.slaney.org/pct/. A tomographic method for determining thebirefringence distribution is described in an article by H. Hammer etal. entitled “Reconstruction of spatially inhomogeneous dielectrictensors through optical tomography”, J. Opt. Soc. Am. A, vol. 22, No 2,February 2005, pages 250 to 255. The full disclosure of these twopublications is incorporated herein by reference.

In a next step, the volume occupied by the last lens L3 on the imageside is subdivided into a multiplicity of volume elements withhomogeneous optical properties, the refractive index n of the volumeelements being dependent on the respective position of the volumeelement in the lens L3. If the material is an optically anisotropicmaterial, then each volume element may furthermore or alternatively beassigned a refractive index ellipsoid whose spatial orientationindicates the direction of the birefringence and whose ratio of major tominor symmetry axes corresponds to the magnitude Δn of thebirefringence. Each volume element may furthermore or alternatively beassigned a transmission coefficient and/or a quantity describing thescattering properties as an additional scalar quantity. In FIG. 2, byway of example, a single volume element (here in the shape of a cube) isindicated by dotted lines and denoted by 48.

In a further step, each volume element is assigned a conjugate volumeelement inside the conjugate volume L3′. For the volume element 48 inthe last lens L3 on the image side in FIG. 2, the volume elementconjugate therewith is denoted by 48′. Each volume element is imaged bythe lens L2, and the part of the lens L3 contributing to the imaging,onto a conjugate volume element; the imaging scale may in this case varylaterally and longitudinally. Distortion-free imaging is not actuallynecessary. The forming of a distorted image of the volume element 48 isschematically illustrated in FIG. 11 which shows how light rays emergingfrom a point within the volume element 48 do not exactly meet at asingle point within the conjugate volume element 48′. Furthermore, theprojection light rays do in fact pass at different angles through thevolume element 48 in the lens L3 than through the conjugate volumeelement 48′, as is indicated by rays 50, 52 in FIG. 2. Nevertheless, forat least partial correction of aberrations it is merely sufficient thatthe same rays pass both through the conjugate volume element 48′ and thevolume element 48 in the lens L3.

FIG. 12 illustrates that, as a result of the distorted image formed ofthe volume element 48, the transmission contrastsT=(I_(max)−I_(mn))/(I_(max)+I_(min)) decreases if an object, here anobject assumed to have a sinusoidal intensity distribution perpendicularto the optical axis and to have a spatial frequency F_(s), isincoherently imaged from the volume element 48 into the conjugate volumeelement 48′. For a good correction effect the contrast T should be atleast 30% for a spatial frequency F_(s)=0.5 line pairs for millimeter(Lp/mm) (e.g., for a spatial frequency of F_(s)=0.7 Lp/mm, for a spatialfrequency of F_(s)=1.0 Lp/mm).

Since the Petzval sum for the imaging may also be nonzero, planesections in the last lens L3 may be imaged with arbitrary and evenvarying curvatures in the conjugate volume L3′. For this reason, theboundary of the conjugate volume element 48′ in FIG. 2 is notcube-shaped, but irregularly curved. The lens L2, which generallyrepresents a more complex optical system, is configured so that thetangential and sagittal image shells have almost the same shape and, inconjunction with oblique spherical aberration, allow to some extent auniform image position both in tangential and sagittal direction.

It is readily possible to determine the conjugate volume elements withthe aid of those known simulation programs which are used in thedevelopment of complex optical systems. It is, however, to be understoodthat the subdivision of the last lens L3 on the image side into volumeelements has been selected here only for reasons of betterrepresentation. In a computational implementation of the methoddescribed above in a computer, it is simplest for the lens L3 to berepresented as a three-dimensional grid network of support points,wherein to each point a set of optical properties measured at therelevant grid positions is assigned. This three-dimensional network ofsupport points is then transformed into the conjugate volume L3′ of thelens L3 with the aid of a transfer function, which describes the imagingof the optical elements lying in-between. A cubic grid then generallybecomes a non-cubic grid of spatially blurred support points. Here,volume centroids can help to define an unambiguously correlated anddistorted grid, despite aberrations of the intermediate image.

In a further step, the conjugate volume L3′ of the last lens L3 on theimage side in the object space is now filled virtually with a number Nof plane-parallel plates. The N plates, which may have been providedalso in an initial design of the projection objective 20, may adjoin oneanother without gaps. By virtual surface deformations of the N plates,an attempt is now made in a computer to correct the wavefrontdeformations which have been caused by different refractive indices inthe volume elements in the last lens L3 on the image side. In order toachieve as complete a correction as possible, the number N of the platesshould initially be selected to be quite large, for example N=20 orN=50.

An analysis is now made as to which of the N plates make no greatcontribution to the correction. These plates may have their surfacesremoved and their optical thicknesses added to neighboring plates, orthey are entirely removed, which may involve a slight adaptation of theother optical elements of the projection objective 20.

The latter case leads to an arrangement of plate-shaped correctionelements, as shown by way of example and denoted by 46 a, 46 b, 46 c, 46d in FIG. 2.

Instead of elements of different thickness, equally thick elements mayalso be arranged at different lengthwise positions along the opticalaxis OA. In the simplest case, all the correction elements are equallythick and are arranged at equal distances from one another.

It should be noted that, in order to be able to address variations inall portions of the last lens L3, the entire conjugate volume L3′ isdesirably devoid of any optical elements. This allows it to be possibleto arrange at any arbitrary axial position a surface of a correctionelement. During the initial design of the projection objective 20,however, all correction plates 46 a to 46 d have to be taken intoaccount in or in the vicinity of the conjugate volume L3′. Since a shiftof the plane-parallel correction elements 46 a to 46 d along the opticalaxis does not alter their optical properties, the final axial positionsof the correction element 46 a to 46 d may be determined once thevariations of certain optical properties in the last lens L3 have beendetermined in the manner as described above.

In order to correct wavefront errors, one or both optical faces of eachcorrection element 46 a, 46 b, 46 c, 46 d may be locally deformed usingconventional techniques. In FIG. 3, in which the correction device 44 isshown on an enlarged scale, such local surface deformations can be seenon the upper side of the correction element 46 a in the enlarged detailand are denoted by 54, 56 and 58.

If the last lens L3 on the image side is anisotropic, then, in a furtheroptimization, the correction device 44 may be supplemented withstructures by which undesired phase differences between orthogonalpolarization states can be corrected. In FIG. 3, these structures areformed as birefringent layers and denoted by 60 a, 60 b, 60 c and 60 d.The birefringent layers 60 a, 60 b, 60 c, 60 d in the correction device44 are applied on the lower side of the correction elements 46 a, 46 b,46 c and 46 d and have a continuous or—as represented in the embodimentshown—discrete thickness distribution, which varies perpendicularly tothe optical axis OA.

FIG. 4. shows an embodiment of a correction device denoted by 144, inwhich the birefringent layers are replaced by form-birefringentstructures 160 a, 160 b, 160 c, 160 d, which are likewise applied on thelower side of correction elements 146 a, 146 b, 146 c and 146 d. Adiffering birefringent effect can be achieved in the form-birefringentstructures 160 a, 160 b, 160 c, 160 d by different dimensioning andarrangement of substructures that form the form-birefringent structures160 a, 160 b, 160 c, 160 d.

FIG. 5 shows a correction device 244 according to another embodiment inwhich plates 260 a, 260 b, 260 c, 260 d for phase difference correctionare arranged so that they are self-supporting between correctionelements 246 a, 246 b, 246 c, 246 d.

Also for correcting the phase difference values, a very large number Mof structures that modify the polarization state of light passingthrough may be assumed initially. In an optimization, those structuresby which only a minor improvement of the imaging properties can beachieved are then gradually removed. The optimization desired for thisis based not on scalar, but on vector calculations.

Thus, with the correction unit 44 it is possible to simultaneouslyachieve a scalar phase correction and also a vector phase differencecorrection in the conjugate volume 48′.

If it is (alternatively or additionally) desirable to correctaberrations which are caused by an inhomogeneous transmissioncoefficient in the last lens L3 on the image side, then it is possibleto use reflective or refractive correction elements whose degree oftransmission or reflection varies locally. The locally varying degree oftransmission or reflection may in this case be accomplished by (anti-)reflective coatings.

In the correction device 144 shown in FIG. 4, vertically aligned arrowsA indicate that the correction elements 146 a, 146 b, 146 c, 146 d, withthe form-birefringent structures 160 a, 160 b, 160 c, 160 d appliedthereon, can be displaced in a vertical direction by manipulators (notrepresented in detail). In this way, on the one hand, fine adjustment ispossible. On the other hand, the correction device 144 may also beadapted retrospectively to modified optical properties in the last lensL3 on the image side. Such changes may, for example, be caused bydegradation phenomena due to the energetic projection light 13.

In the correction device 244 shown in FIG. 5, horizontally extendingarrows B indicate that the correction elements 246 a, 246 b, 246 c, 246d can be removed from the beam path (in a manner not represented indetail) and replaced by other correction elements. In this way, even inthe event of sizeable changes of the optical properties of the last lensL3 on the image side, it is possible to achieve a good correction byusing other correction elements. Exchanging correction elements may beexpedient also if the properties of the last lens L3 change during thelife time of the projection objective 20.

In the embodiment shown in FIG. 2 it has been assumed that thecorrection elements 46 a, 46 b, 46 c, 46 d of the correction device 44are arranged adjacent to one another in the volume L3′, which isconjugate with the volume of the last lens L3 on the image side.

FIG. 6 is a meridional section similar to FIG. 2. through a projectionobjective 320 according to another embodiment. The projection objective320 has four field surfaces, namely the mask plane 22, a firstintermediate image surface 342-1, a second intermediate image surface342-2 and the image plane 28. Between pairs of adjacent field planes afirst pupil surface 343-1, a second pupil surface 343-2 and a thirdpupil surface 343-3 are formed. Optical systems L301, L302, L303, L304,L305 and L306 represented by single lenses are arranged between adjacentfield surfaces and pupil surfaces. The last optical system L306 mayinclude several individual lenses as well, but optionally can containonly one curved lens, for example a plano-concave lens such as shown inFIG. 2, or a meniscus lens. As far as the general design of theprojection objective 320 is concerned, the main difference to theprojection objective 20 shown in FIG. 2 is that it contains one moreintermediate image surface and one more pupil surface.

The intermediate image surfaces 342-1, 342-2 and the pupil surfaces343-1, 343-2, 343-3 may be plane; generally, however, the surfaces areregularly or irregularly curved. With regard to the intermediate imagesurfaces 342-1, 342-2 it should be mentioned that the images formed inthese surfaces may be subject to very significant aberrations. Realisticembodiments of a projection objective having two intermediate imageswill be described further below with reference to FIGS. 8 and 9.

As far as the correction elements are concerned, the projectionobjective 320 differs from the projection objective 20 shown in FIG. 2in that two correction elements 346 a, 346 b formed by plane-parallelplates are arranged such that they are separated by other opticalelements that do not contribute to the correction of aberrationsproduced by the optical system L306. The optical system L304 which isarranged between the two correcting elements 346 a, 346 b may consist ofa single lens, or may include a plurality of lenses and/or other opticalcomponents such as mirrors.

In the last optical system L306 two volume elements 348 a, 348 b areschematically illustrated that are arranged at different distances fromthe image plane 28. Since the third pupil surface 343-3 is located inclose proximity to the last lens system L306, the first volume element348 a is located closer to the image plane 28, and the second volumeelement 348 b is located closer to the third pupil surface 343-3.

The same applies to the first and second conjugate volume elements 348a′ and 348 b′ which are conjugate with the first volume element 348 aand the second volume element 348 b, respectively. More specifically,the first conjugate volume element 348 a′ is located inside the secondcorrection element 346 which is arranged close to the secondintermediate image surface 342-2. The second conjugate volume element348 b′ is contained in the first correction element 346 a, which islocated closer to the second pupil surface 343-2.

Thus the volume elements 348 a, 348 b contained in the last lens systemL306 have conjugate volume elements 348 a′, 348 b′ that are distributedover a larger portion of the projection objective 320. This is aconsequence of the large angles occurring in the last lens system L306,because this implies that different volume elements within thisparticular lens system differ significantly with respect to theirproximity to the image plane 28 and the third pupil surface 343-3.Namely, in volume elements which are located close to the image plane28, light bundles pass through that converge towards a small area in theimage plane 28. In other volume elements located closer to the objectside surface of the last lens system L306, light bundles pass throughthat converge to image points which are distributed over a considerablylarger area.

As a matter of course, additional correction elements may be provided,or optical components contained in the optical system L304 may be usedas correction element. For example, optical surfaces of such opticalcomponents may be provided with non-rotationally symmetric surfacedeformations, or may support (form-) birefringent layers, as it has beenexplained above with reference to FIGS. 3 and 4.

FIG. 7 is a meridional section similar to FIG. 6, a projection objective420 according to still another embodiment. In FIG. 7 componentscorresponding to those shown in FIG. 6 are denoted by the same referencenumerals augmented by 100; most of these components will not beexplained again. The projection objective 420 differs from theprojection objective 320 shown in FIG. 6 only in that the firstcorrection element 446 a is not located in the vicinity of the secondpupil surface 443-2, but in the vicinity of the first pupil surface443-1. The second conjugate volume element 448 b′ is still opticallyconjugate with the second volume element 448 b contained in the lastlens system L406. It should be noted that there are other conjugatedvolumes in the vicinity of a pupil surface which are not suitable forpositioning a correction element which shall correct aberrations causedby the second volume element 448 b. More specifically, conjugated volumeelements have to be separated from a volume element in the last lenssystem L406 by k intermediate image surfaces and k pupil surfaces, withk=0, 1, 2, . . . , N and N being the total number of intermediate imagesurfaces. Otherwise only an inferior correction effect may be achieved,because each intermediate image surface and each pupil surface invertsthe light bundle which emerges from a particular point in the mask plane22 and converges to a conjugate point in the image plane 28. This isexplained in more detail in U.S. Ser. No. 11/570,263 which correspondsto WO 2005/121899 A1 assigned to the applicant. The full disclosure ofthis earlier application is incorporated herein by reference. As amatter of course, the same considerations also apply to the otherembodiments described above.

FIG. 8 is a meridional section through a realistic projection objective520 which has, similar to the projection objective 320 shown in FIG. 6,two intermediate image surfaces 542-1, 542-2 and three pupil surfaces543-1, 543-2 and 543-3. The last lens L523 is made of a CaF₂ crystal.Here it is assumed that this crystal has been grown in a cheap processso that it displays various crystal imperfections resulting ininhomogeneous material-related optical properties. This makes clear thatthe concepts described above may also be applied to lower indexrefractive optical elements having a poorer quality so that significantvariations of certain optical properties occur within the volume of theoptical element.

Volume elements 548 a, 548 b are schematically represented in the lastlens L523 of the projection objective 520. Conjugated volume elementsare denoted by 548 a′ and 548 b′. In this particular embodiment the twoconjugated volume elements 548 a′, 548 b′ are separated by two lenses.Furthermore, the volume elements 548 a′, 548 b′ are contained not inadditional correction elements, but in lenses that are involved inaccordance with the general design of the projection objective 520anyway.

The projection objective 520 is designed as an immersion objective witha numerical aperture NA=1.2. This means that, during the operation ofthe projection exposure apparatus, the interspace between the last lensL523 and the image plane 28 is filled with an immersion liquid 534. Theprojection objective 520 is identical to the projection objective shownin FIG. 3 of WO 2005/111689 which is also assigned to the applicant.

FIG. 9 is a meridional section through a realistic projection objective620 according to a still further embodiment. The projection objective620 is identical to the projection objective shown in FIG. 21 of WO2005/069055 which is also assigned to the applicant.

The projection objective 620 has a first and a second intermediate imagesurface 642-1 and 642-2, respectively, and a first, a second and a thirdpupil surface 643-1, 643-2 and 643-3, respectively. The second pupilsurface 643-2 is formed between two concave mirrors 672, 674, which havespherical surfaces and are arranged between the first and secondintermediate image surfaces 642-1, 642-2 which are located in front ofthe mirrors 672, 674. Immediately in front of the mirrors 672, 674negative meniscus lenses L610, L611 are positioned which are designed astruncated lens elements arranged only at the side of the optical axis OAof the projection objective 620 where the adjacent mirror 672 and 674,respectively, is positioned. Therefore the projection light passes eachmeniscus lens L610, L611 twice.

The projection objective 620 is designed as an immersion objective witha numerical aperture NA=1.2. This means that, during the operation ofthe projection exposure apparatus, the interspace between the last lensand the image plane 28 is filled with an immersion liquid 634.

With the exception of the last lens L620 all lenses are made of quartzglass. The last lens L620 is made of a [111] CaF₂ crystal. Here it isassumed again that this crystal has been grown in a cheap process sothat it displays various crystal imperfections resulting ininhomogeneous material-related optical properties.

Conjugated volume elements 648 a′ and 648 b′ which are conjugate tovolume elements 648 a, 648 b contained in the last lens L620 arecontained in the truncated meniscus lens L611 and the lens L607,respectively. The overall configuration is thus similar to theprojection objective 420 which has been described above with referenceto FIG. 7.

In the following a very straightforward way to determine conjugatedplanes will be explained in more detail with reference to FIG. 10.

FIG. 10 is a meridional section through a projection objective which isdenoted in its entirety by 720. The projection objective 720 includesseven lenses L701 to L707 and has one intermediate image surface 742 andtwo pupil surfaces 743-1, 743-2.

The last lens L707 intersects a plane 770 indicated in broken lines. Theprojection objective 720 contains only one plane 770′ which is opticallyconjugate with the plane 770. The precise axial position of thisconjugate plane 110′, which is arranged between the first pupil surface743-1 and the intermediate image surface 742, may be determined pursuanta certain algorithm. This algorithm makes use of two specific rays,namely a marginal ray 772 and a principal ray 774. The marginal 772 is aray which emerges from a point where the optical axis OA of theprojection objective 720 intersects the mask plane 22. The principal ray774 emerges from a point on the border of the field in the mask plane22. The larger the field which can be imaged is, the further away fromthe optical axis OA is the point where the principal ray 774 emerges.

According to the algorithm mentioned above, the distances D_(m) andD_(p) between the optical axis OA on the one hand and the marginal ray772 and the principal ray 774, respectively, on the other hand aredetermined at the axial position of the plane 770. Then the ratioR=D_(m)/D_(p) is computed. Any plane conjugate with the plane 770 ischaracterized in that at its axial position the corresponding ratioR′=D_(m)′/D_(p)′ is identical (i.e. R=R′). In the projection objective720 this is true for the conjugate plane 770′.

Since the projection objective 720 has only one intermediate imageplane, there is only one conjugate plane for each plane intersecting thelast lens L707. Consequently, there is only one continuous volume whichis conjugate with the volume of the last lens L707. The axial extensionof this conjugate volume is determined by the distance of planes whichare conjugate with planes that intersect the vertices of the last lensL707. By repeating this algorithm for a plurality of planes intersectingthe last lens L707, it is possible to axially resolve the volume of thelast lens L707 with regard to the optical conjugation.

More information relating to the concept of conjugate planes and theconstant ratio R may be gleaned from an essay E. Delano entitled:“First-order Design and the y, y Diagram”, Applied Optics, 1963, vol. 2,no. 12, pages 1251-1256.

The above algorithm is, strictly speaking, only valid in the paraxialregime. Outside this regime, planes have only conjugated (generallycurved) blurred surfaces, as it has been described further above.

The above description has been given by way of example. From thedisclosure given, those skilled in the art will not only understand thepresent disclosure and its attendant advantages, but will also findapparent various changes and modifications to the structures and methodsdisclosed. The applicant seeks, therefore, to cover all such changes andmodifications as fall within the spirit and scope of the disclosure, asdefined by the appended claims, and equivalents thereof.

1. A projection objective comprising: a refractive optical elementhaving a volume and a material related optical property that varies overthe volume, variations of the material related optical property of therefractive index refractive optical element causing an aberration duringuse of the projection objective, a first optical element having a firstoptical surface comprising a first correction mechanism that partiallycorrects the aberration during use of the projection objective; and asecond optical element having a second optical surface comprising asecond correction mechanism that partially corrects the aberrationduring use of the projection objective, wherein: the first correctionmechanism is located in or intersects a first plane which isperpendicular to an optical axis of the projection objective; the secondcorrection mechanism is located in or interests a second plane which isperpendicular to the optical axis of the projection objective; a thirdplane which is perpendicular to the optical axis of the projectionobjective intersects the volume of the refractive optical element; afourth plane which is distinct from the third plane and is perpendicularto the optical axis of the projection objective intersects the volume ofthe refractive optical element; the first plane is optically conjugatewith the third plane; the second plane is optically conjugate with thefourth plane; and the projection objective is a microlithographicprojection objective.
 2. The projection objective of claim 1, wherein arefractive index of the refractive optical element is greater than 1.8at a wavelength of 193 nm.
 3. The projection objective of claim 1wherein the refractive optical element is a last optical element of theprojection objective along a path of light through the projectionobjective.
 4. The projection objective of claim 1, wherein therefractive optical element has at least one curved surface.
 5. Theprojection objective of claim 1, wherein the projection objective isconfigured to be used in immersion operation, and the projectionobjective is configured so that during immersion operation an immersionliquid at least partially covers a photosensitive layer which isarranged in an image plane of the projection objective.
 6. Theprojection objective of claim 5, wherein the refractive optical elementcontacts the immersion liquid during the immersion operation.
 7. Theprojection objective of claim 1, wherein the material related opticalproperty is the refractive index.
 8. The projection objective of claim7, wherein the first correction mechanism comprises a firstnon-axisymmetric deformation of the first optical surface, and the firstnon-axisymmetric deformation is configured to correct a wavefrontdeformation associated with the aberration.
 9. The projection objectiveof claim 8, wherein the first non-axisymmetric deformation comprises anapplied material, and/or wherein the first on-axisymmetric deformationcomprises a region from which material was ablated.
 10. The projectionobjective of claim 8, wherein the second correction mechanism comprisesa second non-axisymmetric deformation of the second optical surface, andthe second non-axisymmetric deformation is configured to correct awavefront deformation associated with the aberration.
 11. The projectionobjective of claim 1, wherein the material related optical property isselected from the group consisting of birefringence, degree ofabsorption, and amount of scattering light.
 12. The projection objectiveof claim 1, further comprising an additional optical element between thefirst and second optical surfaces, wherein the additional opticalelement does not correct the aberration during use of the projectionobjective.
 13. The projection objective of claim 1, wherein theprojection objective has N intermediate image surfaces and N+1 pupilsurfaces, and N is an integer having a value of zero or greater.
 14. Theprojection objective of claim 13, wherein the first and second opticalsurfaces are separated by k intermediate image surfaces, and k is aninteger having a value of at least zero and at most N.
 15. Theprojection objective of claim 1, wherein the refractive optical elementcomprises a fluoride, a chloride or an oxide.
 16. The projectionobjective of claim 1, wherein the refractive optical element comprisesCaF₂.
 17. The projection objective of claim 1, wherein the first opticalelement comprises a first plane-parallel plate.
 18. The projectionobjective of claim 17, wherein the first correction mechanism comprisesa first non-axisymmetric deformation of the first optical surface, andthe first non-axisymmetric deformation is configured to correct awavefront deformation associated with the aberration.
 19. The projectionobjective of claim 17, wherein the second optical element comprises asecond plane-parallel plate.
 20. The projection objective of claim 19,wherein the second correction mechanism comprises a secondnon-axisymmetric deformation of the second optical surface, and thesecond non-axisymmetric deformation is configured to correct a wavefrontdeformation associated with the aberration.
 21. The projection objectiveof claim 1, wherein: a ratio R=Dp/Dm is at least substantially the samein the first and third planes; a ratio R=Dp/Dm is at least substantiallythe same in the second and fourth planes; Dp is a distance between aprincipal ray, which emerges from a point on a border of a field whichis imaged by the projection objective, and the optical axis; and Dm is adistance between a marginal ray, which emerges from a point on theoptical axis in the field which is imaged by the projection objective,and the optical axis;
 22. The projection objective of claim 21, whereinthe ratio R differs in the first and third planes by less than 5%. 23.An apparatus, comprising: an illumination system; and the projectionobjective of claim 1, wherein the apparatus is a microlithographicprojection exposure apparatus.
 24. A method, comprising: manufacturing amicrostructured component using a microlithographic projection exposureapparatus, wherein the microlithographic projection exposure apparatuscomprises: an illumination system; and the projection objective of claim1.